Preparation of phosphorus‐nitrogen flame retardant DOPO‐DBAPh and its flame retarded epoxy‐based composites

Abstract Novel double DOPO‐grafted phthalonitrile flame retardant (DOPO‐DBAPh) has been successfully synthesized via solution reaction with amino‐substituted phthalonitrile monomer, bisphenol A, paraformaldehyde and 9, 10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) as initial materials. The molecular structure of DOPO‐DBAPh was investigated by FT‐IR and 1 H‐NMR spectrum. Then, the DOPO‐DBAPh was added into epoxy (EP) to construct DOPO‐DBAPh/EP fire‐retarding composites by casting forming method. The effect of different proportions of DOPO‐DBAPh/EP fire‐retarding composites on the microstructure, thermal, mechanical, flame retardancy and combustion behavior was investigated. It was found that the DOPO‐DBAPh/EP fire‐retarding composites have a high glass transition temperature ( T g = 156∼179°C), and also have excellent thermal stability ( T 5% = 331.4∼365.4°C) and high flexural strength (96.1∼139.8 MPa). When DOPO‐DBAPh content is 5 wt%, the vertical burning grade and limiting oxygen index (LOI) of DOPO‐DBAPh/EP composites reached V‐0 and 29.6%, respectively. Furthermore, the cone calorimeter testing results showed that the addition of DOPO‐BAPh can obviously inhibit the decomposition rate of cured substance at high temperature and accelerated the formation of dense carbon layer. When DOPO‐BAPh content was 20 wt%, the total heat release rate (THR) and peak heat release rate (PHRR) of the DOPO‐DBAPh/EP fire‐retarding composites were decreased by 38.55% and 35.81% compared with that of cured EP. In a word, this work presents a good way for synthesizing economical and highly efficient flame retardant for use in the preparation of EP‐based fire‐retarding composites with excellent comprehensive performance. Highlights Novel phosphorus‐nitrogen flame retardant (DOPO‐DBAPh) is developed. 5 wt% DOPO‐DBAPh significantly improves the fire safety of EP resin. The DOPO‐DBAPh/EP composites exhibit good thermal and mechanical properties.